These days elevation of the contrast ratio (CR) in liquid-crystal display (LCD) devices is being promoted. In particular, a VA-mode liquid-crystal display device has the advantage that CR in the normal-line direction (hereinafter referred to as “front CR”, and “front CR” may be generally called also as “on-axis contrast ratio”) is high as compared with that in other modes (e.g., IPS, TN and OCB modes), and various studies and developments (for example, for reduction in the line thickness of black matrix, for removal of rib to control the tilt angle of liquid crystal (ribless technology), for improvement of TFT array and electrode slit) are now made for further enhancing the advantage. As a result, in these 6 years, the front CR in VA-mode liquid-crystal display devices has increased from about 400 to about 8000, or by about 20 times. The front CR is an important property that is to be the index of image sharpness, and a VA-mode LCD is characterized by high front CR and is now the mainstream of LCD panels.
On the other hand, in liquid-crystal display devices, it is important that not only the front CR is high but also the panel secures a broad viewing angle (that is, CR in oblique directions (hereinafter this may be referred to as “viewing angle CR”) is high), and that the panel is free from color shift in oblique directions. For improving the viewing angle characteristics of liquid-crystal display devices, in general, a retardation film is arranged on the front side and the backside of the liquid-crystal cell.
For example, in a VA-mode liquid-crystal display device, in general, a retardation film is disposed each one on the front side and on the rear side, thereby sharing the retardation necessary for viewing angle compensation to each one to compensate the viewing angle on the panel. There are known an embodiment where a film having the same retardation is disposed both on the front side and on the rear side (hereinafter this may be referred to as “both-sides retardation film type”); and an embodiment where an inexpensive film such as a plain TAC film or the like is disposed on any of the front side or the rear side, and a film having a large retardation is disposed on the other side (hereinafter this may be referred to as “one-sides retardation film type”).
The former embodiment is advantageous in that one and the same retardation film can be used; and the latter embodiment is advantageous in that a popular film can be used on one side.
As described in the above, in general, a retardation film is used in a liquid-crystal display device for viewing angle compensation of the device; however, heretofore, it has been considered that the retardation property of the film would not contribute toward the front CR at all. It is known that the optical axis misalignment and the haze of the retardation film may lower the front CR of liquid-crystal display device; and it has been considered that, for improving the front CR of liquid-crystal display device, the optical axis misalignment of the retardation film is solved and the haze thereof is lowered (for example, JP-A 2009-139967). Heretofore, it has been found that the light diffusive film to be disposed for viewing angle enlargement is one factor of lowering the front CR; and for preventing the front CR reduction, it is proposed to control the property of the light diffusive film (for example, JP-A 2009-93166).
Japanese Patent 4015840 discloses that, when an optical compensatory film of which retardation-difference between the front direction and the oblique direction is small is disposed between the color filter substrate and the polarizing element in an OCB-mode liquid-crystal display device, then the oblique incident light may be scattered by the color filter, which can prevent the front contrast ratio from lowering. However, the effect of reducing lowering in the front contrast ratio by the use of such an optical compensatory film of which the retardation difference between the front direction and the oblique direction is small could be expected in the case where the retardation film and the polarizing element are disposed so that the slow axis of the retardation film and the transmission axis of the polarizing element are neither parallel nor orthogonal to each other (for example, they cross at 45 degrees), for example, in the case of an OCB-mode liquid-crystal display device; but the effect could not be expected in the case where the retardation film and the polarizing element are disposed so that the slow axis of the retardation film and the transmission axis of the polarizing element are parallel or orthogonal to each other, for example, in the case of a VA-mode liquid-crystal display device. Further, such an optical compensatory film of which retardation-difference between the oblique direction and the front direction is small may be not prepared easily; and the production process and the material for preparing such a film are limited. In recent liquid-crystal display devices showing high-contrast ratio, a low-scattering color filter is built in already, and therefore, the effect of further improving the front contrast ratio of the liquid-crystal display devices showing high-contrast ratio could not be expected for them.